Thermal processing method and apparatus for use with packaging containers

ABSTRACT

A process for thermally treating a product in a container having a headspace above the product, in which the container is subjected to a heated or cooled environment and is simultaneously agitated. The acceleration to which the container is subjected by the agitation is of sufficient magnitude to cause the process to operate in a regime in which the heating or cooling time required for the product to reach a predetermined temperature is very substantially reduced and moreover is substantially insensitive to changes in the acceleration.

BACKGROUND OF THE INVENTION

The invention relates to methods and apparatus for thermally processingpackaging containers, in particular (but not exclusively) metal canscontaining a food product for human or animal consumption.

In a normal canning process the food product is filled into empty cansto an appropriate level, leaving a headspace above the product, the openends of the cans are hermetically sealed with end closures, and then thecans and their contents are sterilised by means of heat. The heatingmedium used is normally either steam or hot water at a temperatureusually of between 115° C. and 130° C. To achieve this temperature thesteam or hot water has to be held at a superatmospheric pressure, andaccordingly it is contained in a pressure vessel known as a retort orcooker.

The cans, after filling and closing, are placed in the retort, theretort is closed, and steam or water is introduced. Temperaturecontrollers are usually present on the retort to maintain the heatingmedium at the desired temperature. While the cans are located in theretort, heat from the heating medium is conducted through the containerwalls and thence passes into the product.

Insofar as their behaviour during thermal sterilisation is concerned,food products are usually classified into three categories, namely: (1)those that heat largely by convection, (2) those that heat largely byconduction, and (3) those that heat by substantial parts each ofconduction and convection. Food products having a very thin consistencyheat largely by convection, that is to say, the heating processgenerates convection currents in the product and these currents dispersethe heat throughout the pack; products of this kind fall intocategory 1. Thick, e.g. relatively viscous or partly particulate,products, heat largely by conduction; for them no significant movementwithin the container occurs, and so heat can substantially only betransferred by conduction; these products form category 2. The productsfalling within category 3, which heat by both conduction and convection,form the smallest of the three categories and include those productswhich either thicken or become substantially more fluid as heatingprogresses.

Because of the need for complete sterilisation all parts of the foodproduct in a can must reach a sufficient temperature for a long enoughtime to achieve so-called commercial sterility. With non-acid (pH>4.5)products which heat largely by convection (i.e. category 1 products)this occurs fairly quickly; for example, a cylindrical can of 73 mmdiameter and 110 mm length typically takes 15-20 minutes in a retort at121° C. to heat to sterilisation temperature ("heat-up time") and remainat that temperature for as long as may be necessary to achievecommercial sterility ("dwell time"). The 15-20 minute period, thus madeup of the heat-up time and any dwell time of the can in the retort atsterilisation temperature, is commonly referred to as the "process"time, which nomenclature will be used hereafter. The process time issubsequent to any time which may be allowed for the retort itself toheat to sterilisation temperature, hereinafter referred to as the"come-up" time. The come-up time may be considerable, e.g. up to halfhour, and some heating of the cans will occur during this time.

The time period formed of the heat-up time of the cans and any come-uptime of the retort is significant in the context of the presentinvention because it represents the time during which the product in thecans is being heated to the sterilisation temperature by heat passingthrough the can wall. This time period, hereinafter referred to as the"heating" time (of the cans), may be supplemented by any dwell time toform the total time during which the cans are subjected to the heatingmedium and which accordingly is hereinafter referred to as the"sterilisation", or more generally, "thermal treatment" time.

It will be seen that, using the definitions given above, thesterilisation time is equal to the process time plus any come-up time;it is also equal to the heating time plus any dwell time.

Category 2 products require much longer heating times than category 1products because of their lesser mobility; a can as described above butcharged with a category 2 product may typically have a process time of80-90 minutes at 121° C., and to this must be added any retort come-uptime allowed and, in addition, the time required for the hot and sterilecan to cool to a predetermined temperature at which it may safely beremoved from the retort. This latter time duration is hereinafterreferred to as the "cooling" time of the can. Thus the total timerequired by the complete sterilisation cycle, i.e. from admission of theheating medium to the end of cooling, may be 2 hours or more; thisoverall time is hereinafter called the "total cycle time".

The long heating times required by category 2 packs (in particular)often lead to overcooking of the product, especially where it liesadjacent to the container wall. In commercial practice it is alreadywell known to reduce the heating time and possible overcooking of acategory 2 food product in a static retort by agitating the can byrotating it whilst in the retort. The rotation of the can has beeneither about its cylindrical axis, or "end-over-end" about a transverse(diametral) axis through its centre. The first form of agitation can begenerated by rolling cans of circular section about their longitudinalaxis, and is used in "Reel" and "Spiral" cookers; however, it is wellrecognised that it does not induce efficient mixing, and the requiredprocess times are reduced by a factor of only about 2. `End-over-end`rotation induces better mixing, and reduction factors in process time ofof 3 or 4 can be expected.

In addition to the commercially used methods described above there areproposals in the patent literature for achieving process time reductionsby agitation. These proposals have variously employed vertical orhorizontal reciprocation (i.e. back-and-forth movement along asubstantially straight path), or angular movement, possibly withreversals, along a circular path, or compound movement having bothreciprocating and angular components.

By way of example, vertical reciprocation is featured in U.S. Pat. No.1,709,175 and German Patent Publication 2031822, whilst horizontalreciprocation is featured in U.S. Pat. Nos. 2,052,096 and 2,134,817, andJapanese Patent Publication JP 56-21584. Angular movement is featured inGB Patent Specification 1593962 at FIGS. 12, 13 and at FIGS. 14, 15,whilst compound movement is featured in FIGS. 16, 17 of GB 1593962 andin French Patent Publication 2096516.

Whilst these and other proposals in the patent literature might beexpected to achieve useful reductions in process time with the attendantadvantages, they contain no indication that the severity of theagitation is important and, moreover, the maximum acceleration given tothe container must exceed a certain minimum value if the process is tobe reliably reproducible whilst achieving high levels of process timereduction.

For example, in the process particularly described in U.S. Pat. No.2,134,817 above, the amplitude of the horizontal reciprocating movementbetween limiting positions (i.e. the peak-to-peak or double amplitude)is said to be usually less than one inch, and the reciprocationfrequency is said to be in the neighbourhood of 140 times (i.e. cycles)per minute.

Assuming a sinusoidal waveform for the reciprocation, these parametervalues given for the process of U.S. Pat. No. 2,134,817 correspond to amaximum value of acceleration of approximately 0.3 times that due togravity (i.e. 0.3 g). As is manifest, however, from the accompanyinggraphs showing the results of tests made by the present Applicants,horizontal reciprocation using accelerations of this magnitude standsonly to achieve a reduction of heating times (in relation to the sameprocess without reciprocation) which is little or no better than thereduction which is commonly achieved by the commercially practicedmethods described above in which the cans are rotated either about theirlongitudinal axes or end-over-end. Moreover, and as will be discussedmore fully later, our tests have indicated that the process described inU.S. Pat. No. 2,134,817 will be subject to wide random variations, andas a result the sterilisation times which would be required in practiceto ensure commercial sterility using that process would have to be madeconsiderably greater than the sterilisation times which can be achieved.

As mentioned above, in the proposal of U.S. Pat. No. 2,134,817 the cansare reciprocated horizontally. Comparative tests performed by thepresent Applicants have shown that horizontal reciprocation offers moreefficient and more uniform product mixing of Category 2 food productsthan does vertical reciprocation, and for this and other reasonshorizontal reciprocation is preferred. For some containers, for example,cylindrical cans which are longer than they are wide, it is advantageousfor them to be generally aligned with the reciprocation path. For othercontainers, however,--for example, squat cylindrical cans--it may bepreferred for them to be orientated differently in relation to thereciprocation path.

The present invention seeks to provide significant and consistentreductions in the heating time of a product in a container in a thermaltreatment process, particularly (but not necessarily) a sterilisationprocess, and accordingly provides, in accordance with a first aspectthereof, a process for thermally treating a product in a containerhaving a headspace above the product, in which the container issubjected to a heated environment and is simultaneously agitated,characterised in that the acceleration to which the container issubjected by the agitation is of sufficient magnitude to cause theprocess to operate in a regime in which the heating time required forthe product to reach a predetermined temperature is very substantiallyreduced and moreover is substantially insensitive to changes in theacceleration. Preferably the acceleration is such that the heating timeof the product is reduced by at least 90% and has a gradient of at most1 min per g of acceleration.

The invention may also advantageously be used for reducing the coolingtime of a product in a container from an elevated temperature to apredetermined lower temperature which may be near or equal to ambienttemperature. In accordance with a second aspect thereof the inventionaccordingly provides a process for cooling a hot product in a containerhaving a headspace above the product, characterised in that thecontainer is subjected to a cooled environment and is simultaneouslyreciprocated with an acceleration of sufficient magnitude to cause theprocess to operate in a regime in which the cooling time required forthe product to reach a predetermined temperature is very substantiallyreduced and moreover is substantially insensitive to changes in theacceleration. Preferably the acceleration is such that the cooling timeof the product is reduced by at least 90% and has a gradient of at most1 min per g of acceleration. Usually the product is thermally treated bya process as defined in the preceding paragraph, and, subsequent to thethermal treatment, is cooled to a lower temperature by the processforming this second aspect of the invention.

The word "headspace" as used above generally has the meaning which isconventional in the food canning industry, that is to say, it denotesthe distance by which the surface of the product in the can falls shortof the top free edge of the can at the end of the product fillingoperation but before the closure is seamed into position to close thecan. In the remainder of the specification and claims this headspace ishereinafter referred to as the "gross headspace", in order todifferentiate it from the actual headspace existing in the can after theend closure has been attached. The latter form of headspace, hereinafterreferred to as "net headspace", is smaller than the gross headspace byabout 3 mm. Thus, for example, a gross headspace of 12 mm may beconsidered to correspond to a net headspace of 9 mm, one of 8 mm to anet headspace of 5 mm, and one of 4 mm to a net headspace of 1 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic depiction of an apparatus for practicing theinvention.

FIGS. 2 to 6 are graphs of time, in minutes, versus acceleration amount,in g's, showing the results of heating tests performed on cans having a9 mm headspace and slurries with bentonite contents of 5%, 7%, 8%, 9%,and 10%, respectively.

FIGS. 7 and 8 are graphs of time, in minutes, versus accelerationamount, in g's, showing the results of heating tests performed on canshaving a 5 mm and a 1 mm headspace, respectively, and slurries withbentonite contents of 7%.

FIG. 9 is a graph of time, in minutes, versus acceleration amount, ing's, showing the results of cooling tests performed on cans having a 9mm headspace and slurries with bentonite contents of 7%.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A method in accordance with the invention for thermally sterilising apackaging container and/or subsequently cooling it to removaltemperature may be performed in an otherwise conventional retort of thekind commonly referred to as "batch" by arranging either for thecontainer to reciprocate in relation to the retort or for the retort toreciprocate and the container to move with it. FIG. 1 diagrammaticallyshows an arrangement of the former kind, in which cylindrical food cansare loaded into wheeled crates 10, 12 which are capable of horizontalreciprocating movement on a track 14 within the thermal enclosure 16 ofa horizontal batch retort. Superheated steam or water (not shown) at atemperature typically of between 120° C. and 130° C. can be supplied tothe thermal enclosure in conventional manner, such as by input andexhaust connections 18, 20.

The crates are coupled together in series by a coupling 22, and ahorizontal drive rod 23 extends from crate 12 through an end wall 40 ofthe thermal enclosure 16, and is attached via a connection block 24 andconnection rod 26 to a crank 28. The crank in turn is coupled to a motor(not shown), and drives the drive rod 23, and thereby the crates 10, 12,to reciprocate horizontally within the enclosure 16 with a cyclicmovement of substantially sinusoidal form. The position, velocity andacceleration of the cans in the crates accordingly each vary with timein a substantially sinusoidal manner.

Accurate alignment of the drive rod 23 with its axis of movement isassured at all times by a bearing block 29 on the thermal enclosure 16,and horizontal guides 30 between which the connection block 24 islocated for movement.

As is represented in FIG. 1 by one can 32, the food cans to be thermallyprocessed are of right-cylindrical form with end panels 34 perpendicularto their axes. Depending on the method of manufacture of the can, one orboth of the end panels may be formed separately from the cylindrical canbody 36. For example, one of the end panels 34 may be a separatelyformed and attached end closure of the "easy-opening" variety, the otherend panel being formed integrally with the body 36. In another type ofcan, both end panels 34 are separately provided by end closures of whichone is attached before, and the other after, product filling.

As is apparent for the can 32 which is individually shown, the cans areloaded into the crates 10, 12 with their axes XX horizontal and alignedor parallel with the drive rod 23. Reciprocation of the drive rodproduced by the associated motor during the sterilisation process maythen cause corresponding reciprocation of the cans along theirlongitudinal axes, as is indicated by the double-ended arrows A. It willtherefore be understood that the reciprocation is normal to the planesof the end panels 34.

After the can contents have reached their sterilisation temperature,i.e. the temperature of the heating medium, they are held in the retortfor as long as is necessary to achieve commercial sterility; the supplyof heating medium to the retort is maintained during this time. Theheating medium is then replaced by a coolant, e.g water at 20° C., andthe cans are cooled to a temperature at which they may safely be removedfrom the retort. Usually they are still at an above-ambient temperatureat this time, 40° C. being typical. After removal the cans are carriedaway for drying, labelling (if necessary), palletising and despatch.

For a commercial process Applicants prefer for the agitation of the cansmentioned above to be maintained throughout the whole sterilisationcycle, most especially when the cans are being heated to, and cooleddown from, the sterilisation temperature, that is to say, during theheating and cooling times respectively; agitation of the cans when attheir sterilisation temperature (i.e. during any dwell time) may be ofonly marginal benefit, but is preferred for process continuity.

The potential effect of agitation on heating and cooling times wasinvestigated by Applicants by means of tests which were performed onconventional, generally cylindrical, metal food cans using an apparatusarranged generally as shown in FIG. 1. The results of those tests arerepresented graphically in FIGS. 2 to 8 for heating, and FIG. 9 forcooling. In all the tests the cans had a height (i.e. axial length) of110 mm, a body diameter of 73 mm, and a body wall thickness of 0.17 mm.The cans were charged with a slurry of bentonite in water to simulatethe food product, the bentonite content of the mixtures being varied (aswill become apparent) in order to simulate products of differentviscosity. In each can a net headspace was left above the product, andthermocouples were located in the product and connected to a measuringand data logging equipment. The thermocouples and their associatedequipment were conventional, and are therefore not shown or described.

Each one of FIGS. 2 to 8 shows the result of a series of six relatedtests performed on cans having the same net headspace and bentonitecontent. In the series of tests depicted in each of FIGS. 2 to 6 thecans tested had an approximately 9 mm net headspace; they were chargedwith slurries having bentonite contents of 5%, 7%, 8%, 9% and 10%respectively. These can all be considered to correspond to category 2food products, that is, products which heat (and cool) largely byconduction. The tests of FIGS. 7 and 8 were performed on cans having a7% bentonite content, respectively with net headspaces of approximately5 mm and 1 mm; they can therefore be usefully compared with FIG. 3, inwhich the bentonite content was the same (7%).

Slurry having a 7% bentonite content has a viscosity which is typical ofmany food products which are encountered in the food processingindustry. A net headspace of 9 mm is also typical of such products, andFIG. 3 was accordingly and appropriately used for the purposes of thedetailed description which follows; however, it should be understoodthat the description applies to each of FIG. 2 and FIGS. 4 to 6 exceptwhere specifically stated.

FIG. 3 shows graphically how the time taken for the simulated product toachieve a predetermined temperature (120° C.) which could be regarded astypical for a commercial sterilisation process was found to vary withthe (maximum) acceleration to which the can was subjected by theagitation performed. Six different graphs are shown, for six differentpeak-to-peak amplitudes of the reciprocating movement; it should beunderstood that by changing the speed of the drive motor the frequencyof oscillation was changed in accordance with the six differentamplitudes so as to achieve the required values of maximum accelerationgiven along the `X` axes of the graphs. For consistency with theremainder of the specification and claims the time taken to achieve thepredetermined temperature is again referred to as the "heating time",although it should be understood that in order to increase the accuracy(and reduce the duration) of the experiments the temperature of theheating medium admitted to the retort was made somewhat greater (130°C.).

From FIG. 3 it is evident that the heating time at zero acceleration,i.e. without any agitation, was approximately 50 minutes, and that foreach of the six peak-to-peak amplitudes used, namely 25 mm, 50 mm, 75mm, 150 mm, 225 mm and 300 mm, the heating time fell sharply as theacceleration was initially increased from this value. For accelerationsof between zero and about 1 g, that is the acceleration due to gravity,the graphs had large gradients and the heating times required were foundto be subject to very substantial random variation. Thus, whilstreductions in the required heating time were possible, in general theywere relatively small; moreover, we believe that the high degree ofinconsistency which could be expected within this range of accelerationswould in commercial practice require substantially longer heating timesto be used than were achieved experimentally; otherwise, there would bethe risk of occasional, but commercially unacceptable, incompletesterilisation of the food product in a can. The potential benefit to beobtained by use of accelerations within this unstable operating regimeis therefore not only limited but also less than it might appear, and itis to be noted that this regime includes the values of accelerationwhich were specifically proposed in the patent specification Nos.2134817 and JP 56-21584 mentioned above.

FIG. 3 further illustrates that as acceleration was increased beyond theinitial unstable regime the graphs flattened out sharply and convergedat an elbow which corresponded to an acceleration in the region of 1 g.Beyond the elbow the graphs had a narrow range of values of heating timeof between 2 and 31/4 minutes, that is to say, less than 10% of theheating time without agitation. Moreover, whilst the heating timesrequired continued to fall with increasing acceleration, the gradientsof the graphs were very much smaller than before and at most one minute(of heat-up time) per g (acceleration due to gravity); the heating timeswere therefore substantially insensitive to changes in the acceleration.Applicants found that the test results were uniform and could beaccurately replicated; thus, not only were the potential reductions inheating time greater than for accelerations within the unstable region,but also those potential reductions could be achieved repeatedly andwith a high degree of confidence. In accordance with the inventiontherefore, Applicants propose that a sterilisation or other thermaltreatment of a food product in a can should be accompanied by agitationof sufficient intensity that the process is conducted within this stableoperating regime.

It will be apparent from FIG. 3 that accelerations of from about 11/4 gand above are suitable for a sterilisation process performed on a foodproduct having a viscosity equivalent to that of a 7% bentonite slurryand a 9 mm net headspace. A preferred range is 11/2 g-21/2 g. Values ofacceleration above 21/2 g would appear to offer little or no significantadditional benefit in the reduction of heating time, but may raiseproblems of a mechanical nature because of the high inertial forceswhich are involved.

FIG. 2 shows graphs corresponding to those discussed above in relationto FIG. 3 but with a 5% bentonite slurry corresponding to a food productof relatively thin consistency and low viscosity. Using the criteriagiven above, the use of accelerations of 3/4 g and above is indicated,accelerations within the range 1 g to 2 g being preferred. It will beseen that the heating time lies within the range of about 11/2 mins. toabout 21/2 mins, i.e. again less than 10% of the heating time withoutagitation. Also, the gradient of the graphs within this stable region issmall, and at most 1 min/g.

FIGS. 4 to 6 show the results of a series of tests similar to that ofFIG. 3 but using simulated food products of 8%, 9% and 10% bentonitecontent respectively. A minimum value of the acceleration of about 11/4g is indicated for FIGS. 4 and 5 and about 11/2 g for FIG. 6,accelerations within the range 11/2 g-21/2 g and 13/4 g-21/2 grespectively being preferred. The gradient of the graphs within thestable region of operation is again 1 min/g at most for each testseries. As with the tests previously described in each series, heatingtimes less than 10% of the heating time without agitation were achieved.

It will be noted from FIGS. 2 to 6 that for the values of accelerationgiving stable operation of the thermal treatment process the heatingtime tended to increase with increasing viscosity of the slurry beingtested. However, reductions in heating time of more than 90% wereavailable throughout the range of viscosities tested, and Applicantsbelieve that, for the most common viscosities encountered in the foodprocessing industry, reductions in heating time of 95% or more can beachieved.

It is also to be noted from FIGS. 2 to 6 that in some of the tests notall of the graphs reliably extended into the stable region of operationwhich forms the basis for the present invention. In the majority ofcases this was because of limitations in the apparatus used for thetests. In particular, using peak-to-peak amplitudes of 25 mm and 50 mm,the apparatus was able to develop values of acceleration of only about3/4 g and 11/2 g respectively; however, although they failed to provideany useful data for the stable region of process operation, these testsnevertheless showed that the unstable region discussed above againoccurred with small amplitude/high frequency agitation.

FIGS. 7 and 8 illustrate the effect of headspace reduction, showingseries of tests on cans having 5 mm and 1 mm net headspace respectively.A 7% bentonite slurry was used to simulate the food product, the samesix amplitudes being used as before. FIGS. 7 and 8 are accordinglydirectly comparable with FIG. 3.

The present invention relies for its effect upon movement of the foodproduct in relation to the can which holds it, and a headspacesufficient to allow such movement to occur is therefore essential. FIG.7 in comparison with FIG. 3 suggests that, in general, a reduction innet headspace of from 9 mm (FIG. 3) to 5 mm has little effect on thepotential reductions in heating time which can be achieved usingaccelerations within the stable region of operation, and that headspaceswithin this regime can therefore safely be used for the purposes of theinvention in a conventional metal can as previously described.

FIG. 8 suggests, however, that a net headspace of 1 mm does not providesufficient room to allow adequate mixing of product within the can, atleast for amplitudes at the extreme ends of the amplitude range used, itbeing seen that the results from amplitudes of 25 mm and 50 mm wereinvalid even for the limited acceleration range which they covered, andthat for 300 mm was invalid until large (i.e. >5.5 g) values ofacceleration were attained. Moreover, the unstable region of operationextended to higher values of acceleration than before. In this respectit should be noted that the net headspaces which are commercially usedfor food packing in cans lie within the range 2-10 mm. For cans havingheadspaces at the lower end of this range intermediate values ofamplitude and acceleration such as 150 mm and 21/4 g seem to beappropriate. However, with suitable choice of the amplitude andacceleration used, reductions in heating time of at least 90% andgradients of at most 1 min/g are again possible.

By increasing the transfer of heat between the can wall and a productwithin the can, the invention can provide significant benefits not onlyin the heating of cans as described above, but also in their coolingfrom an elevated temperature. Usually such cooling of a can will followa heating operation in which the invention is again used. For example, acan containing a food product may be heated to sterilisation temperaturein a retort whilst being agitated in accordance with the teachings ofthe invention to reduce the heating time required; after sterilisationthe can and its contents are cooled by cold water at, typically, 20° C.which is admitted to the retort whilst the retort is again agitated inaccordance with the teachings of the invention so as to reduce the timerequired to cool the can to a lower temperature (typically 40° C.) atwhich it may safely be unloaded from the retort. By such use of theinvention for both the heating-up and cooling-down parts of thesterilisation process, Applicants expect to reduce the total cycle timeto typically less than one quarter of an hour for food products incategory 2; usually, total cycle times of two or more hours would berequired using static conventional sterilisation methods and apparatus.

FIG. 9 illustrates use of the invention in relation to the cooling ofcans of a food product from an elevated temperature such as is used forsterilisation. The same six amplitudes of agitation were used as before,and the cans tested had a 7% bentonite content and a 9 mm net headspace.In those respects FIG. 9 is therefore comparable with FIG. 3. Theelevated temperature was 120° C. (corresponding to the final temperatureachieved in the preceding tests), the temperature to which the cans werecooled being 40° C. From FIG. 9 it will be seen that the cooling timesrequired for the product in the cans to achieve the required temperaturereduction followed curves which were similar to those for the heat-uptime in FIG. 3. Again, an unstable region of operation existed foraccelerations below about 1 g, to be followed by a stable region ofoperation in which further increases in acceleration had little or noeffect on the cooling times which were required; thus, the maximumgradient of 1 minute (of cooling time) per g of acceleration was againfound to apply.

As with operation in a heating regime as previously discussed inrelation to FIG. 3, the cooling treatment of FIG. 9 offered reductionsin cooling time which were at least 90% of the cooling time requiredwith no agitation (43 minutes). It will also be seen from FIG. 9 that aacceleration of about 11/4 g and above is appropriate, 11/2 g-21/2 gbeing preferred.

The reductions in heating time which can be achieved by use of thepresent invention are particularly beneficial for product quality; forexample, Applicants believe that, because the overcooking of category 2products (in particular) can be substantially avoided, it may bepossible to pack in cans products for which criteria of product qualityhave hitherto caused in-container sterilisation to be dismissed as beingunsuitable. The invention offers the possibility of significant costbenefits in e.g. capital equipment and energy; also, because it reducesor eliminates the danger of overcooking of product on the can wall, itenables higher retort temperatures to be used than hitherto, therebyfurther reducing the heating times required.

Whilst being of particular value for the sterilisation of food productsin metal cans, the invention may be applied to thermal processes otherthan sterilisation, to products other than food products, and tocontainers other than cylindrical metal cans. In one possibleapplication the invention is used for the sterilisation of food productsin tray-like containers of a synthetic plastics resin material such aspolypropylene or polyethylene terephthalate (PET). Also, the agitationof containers for the purposes of the invention may be other thanhorizontally directed and rectilinear, and may include a rotarycomponent of motion. Any motion or component is preferably of sinusoidalwaveform (position versus time), although other waveforms may be used.Particularly for cylindrical cans which are of large diameter inrelation to their length it may be preferable to agitate themperpendicularly to their longitudinal axis.

The invention may have application to sterilisation on a continuousbasis, rather than batch sterilisation as is illustrated in FIG. 1.

I claim:
 1. A process for thermally treating a product in a containerhaving a headspace above the product, in which the container issubjected to an environment heated to a predetermined heatingtemperature and is simultaneously agitated, wherein the minimumacceleration amount to which the container is subjected during theagitation is of sufficient magnitude to cause the process to operate ina regime in which the heating time required for the product to reach apredetermined processing temperature is (i) reduced by at least about90% compared to the heating time required to reach the samepredetermined processing temperature when subjected to an environmentheated to the same environmental heating temperature but withoutagitation and (ii) substantially insensitive to increases in themagnitude of the acceleration amount.
 2. A process according to claim 1wherein, within the said regime the heating time of the product falls byat most 1 min per g of change in the acceleration.
 3. A processaccording to claim 1, wherein within the said regime the heating time ofthe product falls by at most 1 min per g of increase in the accelerationamount.
 4. A process according claim 1, wherein the agitation of thecontainers is substantially reciprocation thereof.
 5. A processaccording to claim 4, wherein, the containers are metal cans and thereciprocation of the cans is perpendicular to one or more generallyplane end closures thereof.
 6. A process according to claim 1, wherein,the agitation of the container is substantially sinusoidal (positionversus time).
 7. A process for cooling a hot product in a containerhaving a headspace above the product, wherein the container is subjectedto a environment cooled to a predetermined cooling temperature and issimultaneously agitated with an acceleration amount of sufficientminimum magnitude to cause the process to operate in a regime in whichthe cooling time required for the product to reach a predeterminedprocessing temperature is reduced by at least about 90% compared to thecooling time required reach the predetermined processing temperaturewhen subjected to an environment cooled to the same cooling temperaturebut without agitation and moreover is substantially insensitive toincreases in the magnitude of the acceleration amount.
 8. A processaccording to claim 7 wherein within the said regime the cooling time ofthe product falls by at most 1 min per g of change in the acceleration.9. Apparatus for subjecting a container of a product to a thermaltreatment, the container having a headspace above the product, whichcomprises:(1) enclosure means for providing an environment for thecontainer during said thermal treatment; (2) heating means for admittinga heating medium to said environment to heat the same to anenvironmental heating temperature; (3) support means for supporting thecontainer within the enclosure means; (4) drive means for the containerwhen supported by the support means; the drive means being arranged,while the heating means is effective to heat the environment for saidthermal treatment, to agitate the container with an acceleration amountwhich is of sufficient magnitude to cause the thermal treatment tooperate in a regime in which the heating time required for the productto reach a predetermined processing temperature is reduced by at leastabout 90% compared to the heating time required to reach saidpredetermined processing temperature at the same environmental heatingtemperature but without agitation and moreover is substantiallyinsensitive to increases in the magnitude of the acceleration amount.10. Apparatus for cooling a hot product in a container having aheadspace above the product, which comprises:(1) enclosure means forproviding an environment for the container during said cooling; (2)cooling means for admitting a cooling medium to said environment to coolthe same to an environmental cooling temperature; (3) support means forsupporting the container within the enclosure means; (4) drive means forthe container when supported by the support means; the drive means beingarranged, while the cooling means is effective to cool the environmentfor said cooling, to agitate the container with an acceleration amountwhich is of sufficient magnitude to cause the cooling to operate in aregime in which the cooling time required for the product to reach apredetermined temperature is reduced by at least about 90% compared tothe cooling time required to reach said predetermined temperature at thesame environmental cooling temperature but with agitation and moreoveris substantially insensitive to increases in the magnitude of theacceleration amount.
 11. Apparatus according to claim 9 wherein in orderto agitate the container the drive means is arranged to reciprocate thesupport means in relation to the enclosure means, the enclosure meansbeing stationary.
 12. Apparatus according to claim 9 wherein, theenclosure means is mounted for reciprocation, and the enclosure meansand the support means are horizontally reciprocable together by thedrive means to agitate the container.
 13. A process according claim 4,wherein the reciprocation of the containers is substantially horizontal.14. The apparatus according to claim 11, wherein the reciprocation ishorizontal.
 15. A process for thermally treating a product in acontainer, comprising the steps of:a) filling a container with a productso as to form a headspace in the container above the product; b) placingthe container in an environment that is at an environmental temperature,the environmental temperature being different from the temperature ofthe product filling the container; c) reciprocating the filled containerin the environment so as to impart an acceleration amount to thecontainer of at least about 3/4 g, g being the acceleration due togravity, so as to thermally treat the product.
 16. The process accordingto claim 15, wherein the headspace is greater than 1 mm.
 17. The processaccording to claim 15, wherein the reciprocating motion is applied withan amplitude of at least about 25 mm peak to peak.
 18. The processaccording to claim 15, wherein the reciprocating motion is applied withan amplitude no greater than about 300 mm peak to peak.
 19. The processaccording to claim 15, wherein the product has a viscosity equivalent tothat of a slurry of 5% to 10% bentonite in water.
 20. The processaccording to claim 15, wherein the environmental temperature is abovethe temperature of the product, whereby the product is heated during thethermal processing.
 21. The process according to claim 15, wherein theenvironmental temperature is below the temperature of the product,whereby the product is cooled during the thermal processing.
 22. Theprocess according to claim 15, wherein the thermal treatment furthercomprises bringing the product to a predetermined temperature, andwherein the acceleration amount is of sufficient magnitude to cause thetime required for the product to reach the predetermined temperature tobe reduced by at least about 90% compared to the time required for theproduct to reach the same predetermined temperature in an environment atthe same environmental temperature but without reciprocating motion. 23.The process according to claim 15, wherein the product has a viscosityequivalent to that of a slurry of about 5% bentonite in water, andwherein the container is reciprocated so as to impart an accelerationamount in the range of 1 to 2 g.
 24. The process according to claim 15,wherein the product has a viscosity in a range equivalent to that of aslurry of about 7% to 8% bentonite in water, and wherein the containeris reciprocated so as to impart an acceleration amount of at least about11/4 g.
 25. The process according to claim 24, wherein the container isreciprocated so as to impart an acceleration amount in the range of 11/2to 21/2 g.
 26. The process according to claim 15, wherein the producthas a viscosity equivalent to that of a slurry of about 9% bentonite inwater, and wherein the container is reciprocated so as to impart anacceleration amount of at least about 11/2 g.
 27. The process accordingto claim 26, wherein the container is reciprocated so as to impart anacceleration amount in the range of 13/4 to 21/2 g.
 28. The processaccording to claim 15, wherein the container is subjected to no forms ofagitation other than said reciprocation during step (c).
 29. A processfor thermally treating a product in a container, comprising the stepsof:a) filling a container with a product so as to form a headspace inthe container above the product; b) placing the container in anenvironment that is at an environmental temperature, the environmentaltemperature being different form the temperature of the product fillingthe container; c) reciprocating the filled container in the environmentuntil the product reaches a predetermined temperature, saidreciprocation being applied so as to impart an acceleration amount tothe container equal to at least a critical minimum threshold amount, thecritical minimum threshold amount being that (i) at which the timerequired for the product to reach the predetermined temperature is atleast about 90% less than the time otherwise required to reach the sametemperature without reciprocation and (ii) above which the time requiredto reach the predetermined temperature drops by no more than one minutefor each one g increase in the magnitude of the acceleration amount. 30.The process according to claim 29, wherein the product has a viscosity,and wherein the critical minimum threshold amount depends on theviscosity of the product, with higher viscosities requiring slightlyhigher critical minimum threshold acceleration amounts.
 31. The processaccording to claim 30, wherein the headspace is greater than 1 mm,wherein the reciprocating motion is substantially sinusoidal and isapplied with an amplitude in the range of about 25 to 300 mm peak topeak, wherein the product has a viscosity equivalent to that of a slurryof water and bentonite in the range of about 5% to 10%, and wherein thecritical minimum threshold acceleration amount is at least about 3/4 g.32. The process according to claim 31, wherein the reciprocation isapplied so as to impart an acceleration amount to the container in therange of 1 g to 21/2 g.
 33. The process according to claim 29, whereinthe container is subjected to no forms of agitation other than saidreciprocation during step (c).